1. Field of the Invention
The present invention relates to dynamic-quantity sensors for sensing a dynamic quantity, such as acceleration, angular acceleration, angular velocity, or load.
2. Description of the Related Art
The assignee of this application has proposed an acceleration sensor including a piezoelectric vibrator in Japanese Patent No. 3097464. The acceleration sensor includes a piezoelectric ceramic element for sensing acceleration, a signal processing circuit for processing an output signal from the piezoelectric ceramic element, and a fault self-diagnosing circuit for outputting a self-diagnostic signal to the piezoelectric ceramic element.
Also, an angular velocity sensor is disclosed in Japanese Unexamined Patent Application Publication No. 2002-267448. The angular velocity sensor includes a sensor element including a vibrating part and a sensing part for sensing angular velocity; and a driver circuit for supplying a driving signal to the vibrating part of the sensor element.
Also, the assignee of this application has proposed a dynamic-quantity sensor including a piezoelectric vibrator in Japanese Patent Application No. 2002-326605. Japanese Patent Application No. 2002-326605 corresponds to U.S. patent application Ser. No. 10/329,507 filed on Dec. 27, 2002. The dynamic-quantity sensor includes two piezoelectric vibrators to which stresses generated by a dynamic quantity are applied in opposite phases, a voltage signal applying circuit for applying a common voltage signal to the piezoelectric vibrators, a current-voltage converting circuit for converting current signals flowing in the piezoelectric vibrators into voltage signals, and a phase-difference signal processing circuit for detecting a phase difference between the voltage signals output from the current-voltage converting circuit and for outputting a dynamic quantity sensing signal.
An example of the structure of the dynamic-quantity sensor described in Japanese Patent Application No. 2002-326605 will now be described with reference to FIG. 8. The dynamic-quantity sensor includes an acceleration sensing element 10, a current-voltage converting and signal adding circuit 11, a feedback signal processing circuit 12, a phase-difference voltage converting circuit 13, and an amplification and filter circuit 14.
The acceleration sensing element 10 includes piezoelectric vibrators Sa and Sb to which stresses generated by a dynamic quantity are applied in opposite phases. The piezoelectric vibrators Sa and Sb are connected in series with resistors RLa and RLb, respectively. The current-voltage converting and signal adding circuit 11 converts current signals flowing in the piezoelectric vibrators Sa and Sb into voltage signals to be output as an Sa signal and an Sb signal, respectively. Also, the current-voltage converting and signal adding circuit 11 outputs a combined signal created from both signals.
The feedback signal processing circuit 12 amplifies the voltage of the combined signal, limits the amplitude, and outputs a voltage signal Vosc to the acceleration sensing element 10. The voltage signal Vosc is applied to a node between the piezoelectric vibrators Sa and Sb.
The phase-difference voltage converting circuit 13 generates a voltage signal that is proportional to a phase difference between the Sa signal and the Sb signal, which are converted to the voltage signals.
The amplification and filter circuit 14 amplifies the voltage signal converted by the phase-difference voltage converting circuit 13 with a predetermined gain and eliminates components in an unwanted frequency range to output an acceleration sensing signal.
In the circuits shown in FIG. 8, both the resonant frequencies of the piezoelectric vibrators Sa and Sb are adjusted to be equal to the frequency of the voltage signal Vosc and stresses in different phases, namely, compression (or tension) and tension (or compression), are applied to the piezoelectric vibrators Sa and Sb, respectively, so that an output signal is captured from the amplification and filter circuit 14.
A circuit for determining the occurrence of a fault in accordance with a change of a sensor output by sending a self-diagnostic signal to a piezoelectric ceramic element is described in Japanese Patent No. 3097464. Also, a structure in which a self-diagnosis is performed by applying a signal synchronized with a sensor element driving signal upstream of a demodulator is described in Japanese Unexamined Patent Application Publication No. 2002-267448. For example, if a dynamic-quantity sensor is used for vehicles, such a self-diagnostic function is essential for ensuring high reliability.
In the dynamic-quantity sensor described in Japanese Patent Application No. 2002-326605, however, current signals flowing in two piezoelectric vibrators to which stresses generated by a dynamic quantity are applied in opposite phases are converted into voltage signals and self-oscillation occurs due to a feedback circuit. Thus, the self-diagnostic circuit used in Japanese Patent No. 3097464 or Japanese Unexamined Patent Application Publication No. 2002-267448 cannot be used.